1. Field of the Invention
This invention relates to improvements of an engine comprising: a fuel injection valve in the air intake system; wherein the air-fuel ratio is controlled within a predetermined narrow range through the control by said fuel injection valve, and particularly to a fuel injection control method for controlling the air-fuel ratio of the engine constantly within a predetermined narrow range throughout all operating conditions and an apparatus therefor.
2. Description of the Prior Art
A three-way catalyzer can simultaneously purify HC, CO and NOx that are noxious components contained in exhaust gases. However, in order to effectively use this sort of catalyzer, it is necessary to control the air-fuel ratio of the engine very accurately and to maintain the value thereof within a predetermined narrow range throughout all operating conditions.
FIG. 1 illustrates the outline of the fuel injection control unit of the engine equipped with a purifying unit having a three-way catalyzer. Denoted at reference numeral 1 is the main body of the engine, 2 an intake air flow sensor, 3 an air intake pipe, 4 a throttle valve, 5 a negative pressure sensor mounted on said air intake pipe 3, 6 a fuel injection valve, 7 an ignition distributor, 8 an exhaust pipe, and 9 an exhaust gas purifying unit having a three-way catalyzer which is mounted at the intermediate portion of the exhaust pipe. Denoted at 10 is a fuel control circuit which receives a signal corresponding to the intake air flow rate detected by the intake air flow sensor 2 and a signal corresponding to the RPM of the engine detected by the ignition distributor 7 so as to control the fuel injection valve 6 to maintain the air-fuel ratio within a predetermined range.
If the quantity of the intake air per unit time is Q and the fuel injection quantity per unit time is q, the air-fuel ratio A/F of the engine is given by the following formula. EQU A/F=Q/q (1)
If the fuel injection valve 6 injects once per rotation of the engine, one injection time (width of a injection pulse) is T and the RPM of the engine is N, the relationship therebetween is given by: EQU q.varies.TN (2)
If A/F in the formula (1) is K, the formula (1) is changed into q=KQ and q=KQ is substituted into the formula (2), the folliowing formula is obtained. EQU T.varies.K.multidot.Q/N (3)
In other words, the control circuit 10 produces an ouput of a pulse width T proportional to Q/N commensurate to the RPM of the engine so as to maintain the air-fuel ratio at a certain value.
Now, during the accelerating or decelerating condition of the engine, the air-fuel ratio is deviated from the controlled air-fuel ratio due to the lags in the operations of the air intake system and the exhaust system. In other words, the negative pressure in the air intake pipe rapidly approaches the atmospheric pressure in the rapidly accelerating condition of the engine, whereby only a part of the fuel injected into the air intake pipe is vaporized and the fuel sucked into the cylinders of the engine is decreased, thus resulting in a lean air-fuel ratio. In contrast with this, in the rapidly decelerating condition of the engine, the pressure in the air intake pipe approaches the vacuum pressure and the liquid fuel in the air intake pipe is vaporized in large quantities thus resulting in a rich air-fuel ratio. To cope with said lags in the operations of the air intake system and the fuel system, with the conventional fuel injection control unit, such countermeasures have been taken that the accelerating or decelerating condition of the engine is detected by, for example, differentiating the intake air pressure or the opening degree of the throttle valve, and in the case that this differential value is larger than a predetermined value, i.e., the transitional fluctuating value becomes large to a certain extent, the fuel is increased or decreased commensurate to the transitional fluctuating value, thereby correcting the air-fuel ratio. However, even with the fuel injection control unit of the type described, it has been difficult to control the air-fuel ratio within a predetermined range due to the lag in the operation of the control system during the accelerating or decelerating condition of the engine. Therefore, with the conventional fuel injection control unit further improved, the correcting value corresponds to a value commensurate to a transitional fluctuating value detected until the transitional fluctuating value during the accelerating or decelerating condition of the engine reaches the peak value, and to a value decreased from the peak value at a definite time constant after the transitional fluctuating value has reached the peak value. Even with this conventional fuel injection control unit further improved, there has still remained such a disadvantage that the air-fuel ratio is fluctuated by the RPM of the engine.